This invention relates to pneumatically operated diaphragm pumps and, more particularly, to a method and apparatus for avoiding icing and/or stalling.
Pneumatically driven pumps are well known for their utility and frequently utilize either double acting pistons or diaphragms to alternately compress and expand pump chambers to force the exit of the fluid from one chamber while inducing the entry of additional fluid into the other chamber. Since pneumatically driven pumps do not require an electric or internal combustion engine to drive the pumping chambers, such pumps are particularly useful in locations where combustible or explosive materials are present.
One of the problems generally associated with pumps of this type is icing. The actual air flow patterns through the valves are both transient and highly turbulent as a consequence of cyclic operation of the air distribution valve to effect repeated openings and closings of valve exhaust ports. The air jets through the air valve passages are at times at very high Reynolds numbers and hence in the turbulent flow range. Associated with such highly turbulent flows are both velocity and pressure fluctuations, the mean-square pressure energy of which can approach the magnitude of the operating pressures.
Whenever a gas is expanded from a higher pressure to a lower pressure, a cooling of the gas takes place and internal energy is released, the equation relating pressure (P), velocity (V) and temperature (T) of the gas before (i.e., at time 1) and after expansion (i.e., at time 2) being as follows: EQU P.sub.1 V.sub.1 /T.sub.1 =P.sub.2 V.sub.2 /T.sub.2
In the typical three-way air valve used in controlling the operation of such pumps, P.sub.1 and P.sub.2 have time-dependent mean values and P.sub.2 is further subject to severe turbulent fluctuations about the time-mean pressure values. When the valve is operated in environments of low ambient temperatures and high moisture content, icing conditions often develop.
Known prior art pumps have attacked the problem of ice formation by incorporating an air dryer to remove moisture from the air supply system. However, air dryers are often extremely expensive and only marginally successful in climatic conditions of low temperature and high humidity. The additional drop in operational pressure through the air dryer may also be undesirable.
Others, such as those disclosed in Rosen et al. U.S. Pat. No. 3,635,125 dated Jan. 18, 1972, have provided flexible mufler plates and placed a thermal barrier between the valves and the exhaust ports. Others such as the Nord et al. U.S. Pat. No. 3,176,719 dated Apr. 6, 1965, have sought to physically displace the exhaust ports from the pump. Still others such as the Phinney U.S. Pat. No. 2,944,528 dated July 12, 1960, have used oscillating reeds in the exhaust valve or cavity.
Still another known approach to this icing problem is the use of chemical deicing agents such as ethyl alcohol and ethylene glycol. However, these chemical deicing agents are often marginally successful and also introduce an undesirable environmental condition in introducing ethyl alcohol and ethylene glycol vapors into the ambient air.
In still other known dual diaphragm pumps such as that disclosed in the Budde U.S. Pat. No. 4,406,596 dated Sept. 27, 1983, the two operating air chambers are connected to reduce the pressure level of the air being exhausted.
In one aspect of the present invention, icing is reduced by the controlled bleeding of high pressure air from an internal high pressure chamber to an internal low pressure chamber. The high pressure air furnishes internal energy and thus velocity to the exhaust air and thus mechanically displaces ice as it forms. This air by-pass provides a stepdown release of the motive gas, i.e., it reduces the pressure drop across the valve by increasing the pressure in the low pressure chamber and increases the pressure drop across the outlet aperture to increase exit velocity as indicated above.
Pneumatically operable pumps typically use a source of compressed air which is distributed by a reciprocating three-way valve to drive the pistons or diaphragm in the pumping chambers. Known valves such as described as prior art in the Wilden Pat. No. 3,071,118 generally require lubrication with an oil mist because the metal piston travels in a metal cylinder. The clearance required between such metal parts prevents a tight seal, allowing a high amount of air leakage, making it inefficient. However, the use of an oil mist is undesirable in many applications because of the contamination of the atmosphere and material such as foodstuffs being pumped.
Another known type of control valve such as disclosed in the aforementioned patent to Budde uses a metallic piston with a resilient plastic compression seal which eliminates the need for lubrication. While such resilient piston seal rings or o-rings create a barrier that prevents leakage of the compressed air between the piston and the piston wall, the use thereof in many cases is not cost effective due to the frequency of replacement of the seal rings. Generally, the rings fail because the actual contact surface is extremely small compared to the diameter and weight of the piston, uniformly for vertical piston rings but uneven on the lower part of the ring for horizontal pistons as a result of the force of gravity.
In another aspect, the present invention eliminates the maintenance problems of oil mist free valves by forming the piston seals integrally with the piston of a suitable plastic material such as polytetrafluorethylene (PTFE) or the like. In this way, the contact surface area may be increased relative to the diameter and weight of the piston.
Another problem associated with double diaphragm pumps is the potential for stalling. Stalling is prevented in the present invention by the use of a pilot valve cylinder resiliently deformable under pressure so that air can be bled from a selected one of the potentially opposing chambers of the air distribution valve to thereby ensure operation. In addition, the bleeding of air from a selected valve chamber may be used to slow the speed of reciprocating movement of the air distribution valve piston during the terminal part of a movement thereof. This reduces the impact of the piston on the end walls of the cylinder and thus reduces the potential deformation and sticking of the piston to the end wall.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art from the claims, and from the following detailed description when read in conjunction with the appended drawings.